First, the relationship between cell surface histocompatibility (H) antigens and immunogenicity was investigated in a series of B cell leukemias of inbred strain 2 guinea pigs. It was found that the presence of certain H antigens, Ia antigens on the cell surface were necessary for the immunogenicity of the tumor associated transplantation antigens (TATA). A leukemic line that lacked this Ia antigen was not immunogenic despite the fact that it could be demonstrated to possess the TATA. Second, Ia antigens are being detected on human leukemic cells. Human leukemic cells are being labelled internally with radioactive amino acids. The cells are then solubilized and the glycoproteins are absorbed to and eluted from lectin columns and then run on polyacrylamide gels. Molecules presumed to be Ia antigens can be detected by their characteristic mobility on the gels. In addition the cell extract can be precipitated by known alloantisera to human Ia antigens to determine whether the peaks observed gel electrophoresis are actually Ia antigens.